The present invention relates to an oscillator used to drive an inductive sensor. In particular, the present invention relates to detection systems such as a vehicle detector which use inductive sensors.
Inductive sensors are used for a wide variety of detection systems. For example, inductive sensors are used in systems which detect the presence of conductive or ferromagnetic articles within a specified area. Vehicle detectors are a common type of detection systems in which inductive sensors are used.
Vehicle detectors are used in traffic control systems to provide input data required by a controller to control signal lights. Vehicle detectors are connected to one or more inductive sensors and operate on the principle of an inductance change caused by the movement of a vehicle in the vicinity of the inductive sensor. The inductive sensor can take a number of different forms, but commonly is a wire loop which is buried in the roadway and which acts as an inductor.
The vehicle detector generally includes circuitry which operates in conjunction with the inductive sensor to measure changes in inductance and to provide output signals as a function of those inductance changes. The vehicle detector includes an oscillator circuit which produces an oscillator output signal having a frequency which is dependent on sensor inductance. The sensor inductance is in turn dependent on whether the inductive sensor is loaded by the presence of a vehicle. The sensor is driven as a part of a resonant circuit of the oscillator. The vehicle detector measures changes in inductance in the sensor by monitoring the frequency of the oscillator output signal.
Examples of vehicle detectors are shown, for example, in U.S. Pat. No. 3,943,339 (Koerner et al.) and in U.S. Pat. No. 3,989,932 (Koerner).
In the past, vehicle detectors have typically used constant voltage resonant type oscillators, such as Colpitts, Pierce, or positive feedback logic inverter oscillator circuits. An example of a Colpitts oscillator used as the sensor drive oscillator in a vehicle detector is shown in FIG. 14 of the Koerner et al U.S. Pat. No. 3,943,339.
The inductive sensors connected to a vehicle detector can have a nominal inductance which varies significantly. In addition, the inductive sensors can be located at varying distances from the vehicle detector, which results in variation in the sensor resistance and inductance contributed by the lead-in cables which connect the sensor to the vehicle detector.
It is extremely desirable that the current supplied to the inductive sensor be sinusoidal with minimal distortion. The presence of distortion can affect the accuracy of the oscillator frequency measurements, which are based on the fundamental frequency of the oscillator signal.
The prior art sensor drive oscillators have been capable of providing sine-wave oscillation for some load ranges, but they are not capable of providing sinewave oscillation over the range of loads described above. Increased distortion leads to increased instability in the ability to discern the fundamental frequency of the oscillator.
In addition, with the prior art oscillators, the inductive sensor is driven with a constant voltage. The larger the inductance of the sensor, and the higher the resistance of the lead-in cable to the sensor, the lower the value of drive current supplied to the sensor. This is significant because the sensitivity of the inductive sensor can be a function of the drive current as well as the location of the object being sensed. A change in the drive current being supplied to the inductive sensor can change the magnitude of the apparent inductance changes exhibited by that sensor for a particular vehicle.
An improved sensor drive oscillator which is capable of providing sine-wave oscillation over a wide range of inductive loads, which can be started and stopped quickly, and which provides consistent drive current for a number of different sensor configurations with different inductances and losses would be highly desirable.